Gillian Flynn’s novel “Gone Girl” isn’t really a whodunit (or, more accurately, a “whathappened”). It’s a skeptical, even cynical look at relationships in the guise of an airport novel — a page-turner where the real meat lies in between the many hairpin turns. The movie is more of a thriller than a relationship movie. It does the inverse of the source: Where the book was better in its first half and more of a trashy twist-a-thon in its second, the film, directed by David Fincher, actually gets better once it gets trashy. And it’s not just trashy; it’s phantasmagoric.

Oddly, Flynn is partly responsible for that switch: She wrote the script herself, gutting her work down to a still-epic length yet retaining the structure. As in the book, we hopscotch between Ben Affleck’s ex-journo, Nick Dunne, in the present, as he participates in the hunt for his freshly AWOL wife, Amy (Rosamund Pike), and the past, which animate Amy’s diaries, starting from when she and Nick met-cute at a party. It loses Nick’s narration, but Affleck’s weariness and occasional temper tell us all we need to know as the investigation gradually turns his way, suggesting him of foul play. Meanwhile, Amy tells her story: of a woman who fell for a cool, charming guy who turned out to be shlubbier and less faithful than he let on, especially once a recession and new media killed both of their journalism careers and forced them from Brooklyn to his nowhere, Missouri hometown.

Also as in the book, the film is neatly divided into two halves, separated by a shocking (although, at least in the book, gradually foreseeable) twist. Flynn’s novel loses something at this point, devolving into a series of sometimes questionable (and yet always highly, addictively, gigglingly readable) plot turns. With the movie, it’s this point where it finally comes alive. It handles the present day Nick business well but fumbles the Amy flashbacks. Flynn’s book paints a heartbreaking portrait of failure, disappointment and loneliness amidst a crumbling marriage, but her script and Fincher’s direction rush through it. Fincher even cranks up Trent Reznor and Atticus Ross’ droning synth score where you can barely hear Nick and Amy’s banter-y dialogue as they fall in love, or their arguments when they fall out of it.

But once the plot twist — which of course we won’t divulge — kicks in, Fincher suddenly figures the material out and the film starts cooking. What he figures out is that he’s not so interested in this as a pessimistic study of marriage and intimacy, but as an unnaturally crazy piece of mega-trash. What Fincher brings to “Gone Girl,” beyond his usual technical precision and sculpting of performances, is, for one, his underrated, mordant sense of humor. Along with the outside-the-box casting of Tyler Perry, as Affleck's hot shot shyster lawyer, and Neil Patrick Harris, as one of Amy’s creepy exes — as well as the uncannily perfect use of Affleck as a guy everyone hates — there’s also subtly amusing bits of pure filmmaking. Very funny is the cut from Nick and Amy’s engagement kiss to Nick, in the present, giving a saliva test to the police. (Even the surprise blink-and-you-miss-it cameo of a certain major movie star’s shlong is just there for kicks.)

Fincher’s other major contribution is a simple one: the promise that he will neither eliminate nor hold back on Flynn’s nuttier conceptions. He also retains the strange empathy Flynn has for one otherwise highly hissable character — an action that softens what can be argued (and has, tirelessly) as a sexist creation. (This is a tough film to discuss while avoiding spoilers. Like the novel, it has, shall we say, problems worth angrily debating.)

You can barely see Ben Affleck in one of the many times David Fincher's "Gone Girl" intriguingly underlights its shots.Credit: Merrick Morton

That Fincher has delivered an unlikely adaptation of a novel that the world has already devoured is not surprising. He’s a director with unusual obsessions, who read “The Girl with the Dragon Tattoo” but clearly had no interest in the mechanics of the (pretty stupid) plot. Instead, his film version did a “Zodiac”-lite on the gruntwork of the investigation aspect, then got so bored with itself during the moronic climax that Fincher had to crank up Enya just to keep himself amused. He doesn’t seem particularly interested in the relationship angle in “Gone Girl,” but he clearly loves the idea of deception and self-deception, and the way people tell stories to convince others, and themselves, of what’s true. (Nick is always talking about how the truth of his innocence will ultimately exonerate him, to which Perry’s lawyer rolls his eyes.)

And of course he gets really goosed at how insane Flynn’s tale gets in its second half. He’s fully committed to turning this into a highly polished, meticulously crafted pile of junk cinema, one that keeps one-upping itself, piling one shock upon another until it climaxes with one of cinema’s craziest actual climaxes. It’s not that it gets dumb: In fact Fincher and Flynn’s script carry over the complexity from the novel and the film’s first half, where characters are various shades of gray and truth is always mutable. For a good chunk “Gone Girl” seems like an unexpected mismatch of material and director. It still is a mismatch, but it’s a mismatch in eventually productive ways, turning the film into an experience nearly as memorable as the book, if in a way few probably imagined.